Polarization handling, including splitting and rotation of optical modes, is an important topic in integrated optics, including systems that transmit optical signals over optical fibers.
For coherent transmission, dual polarization division multiplexing (DPDM) helps to increase the bandwidth by a factor of two. DP-QPSK is one of the most important modulation mechanisms for long-haul coherent transmission. A polarization splitter and rotator (PSR) is one of the fundamental building blocks of a DP-QPSK transceiver. In order to launch two polarizations from a photonic integrated circuit (PIC) to an optical fiber, a PSR is required to multiplex polarizations on the transmission (TX) side. A PSR can also de-multiplex polarizations at the receiver (RX) side to ensure the PIC receives only light of a single polarization.
Also known in the prior art is Thompson, U.S. Pat. No. 5,493,624, issued Feb. 20, 1996, which is said to disclose an integrated optics polarization state converter that comprises optically in series a first TM0 to TM1 mode converter that is substantially transparent to TE0, a concatenation of total internal reflectors and a second TM0 to TM1 converter, similarly substantially transparent to TE0, which is connected the way round so as to operate as a TM1 to TM0 converter. Each TM0 to TM1 converter may comprise a tandem arrangement of a 2×2 TE0/TM0 polarization beam splitting coupler and a mismatched, 3 dB maximum, 2×2 beam splitting coupler. The place of the TM0 to TM1 converters substantially transparent to TE0 may be taken by TE0 to TE1 converters substantially transparent to TM0.
Also known in the prior art is Roth, U.S. Pat. No. 8,855,449, issued Oct. 7, 2014, which is said to disclose embodiments of an invention that enable polarization diversity using a more general component than current polarization splitter and rotator solutions. Devices such as an optical receiver, transmitter or duplexer may utilize polarization diversity to efficiently process incoming signals regardless of the signal's polarization. Embodiments of the invention may be described as enabling polarization diversity via an adiabatic waveguide polarization converter. When utilized in an optical system of discrete components or in a photonic integrated circuit (PIC), this adiabatic waveguide polarization converter may receive an unknown single-mode polarization of light. This light may, for example, originate from a remote location and come through a single mode fiber. As described in further detail herein, embodiments of the invention reduce the requirements and component complexity for polarization handling for polarization diversity systems. By reducing the component complexity, insertion loss is reduced, device footprint is reduced, and device reliability and tolerances may be improved.
TM0-TE1 tapers have been reported in publications such as D. Dai and J. E. Bowers, “Novel concept for ultracompact polarization splitter-rotator based on silicon nanowires,” Opt. Express 19, 10940-10949 (2011) and D. Dai, Y. Tang, and J. E. Bowers, “Mode conversion in tapered submicron silicon ridge optical waveguides,” Opt. Express 20, 13425-13426 (2012), but those adiabatic tapers usually have long device lengths.
Y-junctions used to split both TE0 and TE1 has been reported in in various publications, including the Dai and Bowers 2011 paper and Y. Ding, H. Ou, and C. Peucheret, “Wideband polarization splitter and rotator with large fabrication tolerance and simple fabrication process,” Opt. Lett. 38, 1227-1229 (2013).
There is a need for improved integrated polarization splitters and rotators.